Magnetic resonance imaging (MRI) is a well-established diagnostic imaging technique that is useful in many applications, due to its capacity to produce superior soft tissue contrast compared to other diagnostic imaging techniques. MRI is also suitable for use with a wide variety of contrast mechanisms, such as T1, T2, proton density, diffusion tensor imaging, or diffusion weighted imaging, that can reveal both subtle and dramatic anatomical, functional, and pathological details with higher sensitivity and specificity than other imaging techniques.
Recently, MRI is being employed in image guided applications, such as radiation therapy. For example, in image guided radiation therapy, the ability to visualize active tumours and real-time radiation dose distributions is expected to improve targeting of radiation doses to tumour regions and reduce radiation exposure to surrounding healthy tissue. This is expected to result in more efficient treatments and higher survival rates in patients undergoing radiation therapy. Further, in some cases, image guided radiation therapy is the only viable treatment for certain types of cancer.
In image guided applications, geometrical accuracy of the MRI equipment is considerably more important than in diagnostic imaging applications, where modest image distortion does not affect diagnostic outcomes. Image guided applications typically require accuracy, from the MRI equipment, at the millimeter level. By contrast, diagnostic imaging applications can generally tolerate image distortions at the centimeter level.
Quality assurance devices, called MRI geometric distortion phantoms, are used to assess the level of geometric distortion in MRI systems. Geometric distortion may be caused by system imperfections in MRI equipment components, such as the gradient coils or the main BO magnetic field coils. MRI geometric distortion phantoms, or MRI phantoms, may be used for quality assurance purposes, such as tracking system performance over time, or they may be used by OEM vendors for testing purposes, such as quantifying errors in the image geometry.
MRI phantoms have a known 2D or 3D geometry and contain a MRI signal producing material within a MRI invisible container. Typically, the MRI signal producing material is a liquid and the MRI invisible material is a plastic. The container may also contain either intersecting grid structures of known dimensions or fiducial markers distributed in known positions. The MRI signal producing material and/or other markers are then detected using the MRI system and the resulting image is compared against the known geometry of the MRI phantom to calculate image distortion in the MRI system.
Geometric accuracy of MRI phantoms can be compromised by pressure changes caused by the difference in volume thermal coefficient of expansion of the materials used to construct and fill the MRI phantom. Based on typical values for the volume thermal coefficient of expansion for commonly used materials (mineral oil and acrylic), the change in pressure per degree Celsius (ΔP/° C.) is about 60 kPa/° C. This is a significant increase in the internal pressure of a sealed vessel, such as a MRI phantom, and can result in geometrical distortion to the MRI phantom itself, thereby affecting the accuracy of the MRI phantom in measuring geometrical distortion of MRI equipment. In addition, the increase in pressure may result in liquid rupturing from the MRI phantom if the MRI phantom is subjected to a wide fluctuation in temperature and/or ambient pressure, such as during shipping.
Accordingly, there is a need for a MRI phantom that is capable of maintaining its structural integrity and geometrical accuracy when subjected to fluctuations in temperature and/or pressure.